Bone block breaks top layer bedrock.

Put the summary of the bug you're having here

What I expected to happen was...:
There should be a continuous layer of bedrock on the roof of the nether.

What actually happened was...:
I was flying on the roof of the nether when I saw a block out of the ordinary. It appears that the fossil structure that generated has replaced the bedrock.

Steps to Reproduce:
1. create a world with the seed "6088286447899686073"
2. go to the nether
3. go to coordinates 400 128 -123